In adaptive modulation methods used in wireless communication, a modulation scheme of a modulator in a transmitting station and a modulation scheme of a demodulator in a receiving station are sequentially switched in accordance with modulation scheme information transmitted from the transmitting station to match the modulation schemes (more strictly speaking, with respect to the demodulator in the receiving station, a demodulation scheme corresponding to the modulation scheme of the modulator in the transmitting station; this is also applied to the following description) with each other, and communication is performed. As a result of such a process, the modulation scheme of a transmission line is switched without an instantaneous interruption. However, if a wireless line is interrupted (if a line disconnection has occurred due to, for example, bad weather), the modulation scheme information from the transmitting station does not arrive at the receiving station. For this reason, it is difficult to match the modulation scheme of the modulator in the transmitting station with the modulation scheme of the demodulator in the receiving station. For this reason, in adaptive modulation methods relating to the present invention, it is common to make the modulation scheme of the modulator in the transmitting station and the modulation scheme of the demodulator in the receiving station the same modulation scheme that provides the smallest number of levels. This is because the modulation scheme that provides the smallest number of levels is tolerant to deteriorations of lines. As a result of such control, the above-described adaptive modulation methods make it easy to perform restoration from a disconnection of a wireless line.
Moreover, an apparatus which improves the transmission quality by providing a plurality of lines from a transmitting station to a receiving station has been proposed (see Patent Document 1). The transmitting station disclosed in Patent Document 1 transmits, over the plurality of lines, a signal that has been obtained by copying data to be transmitted and modulating the resultant data with the same modulation scheme. The receiving station selects a line providing the high quality from among the plurality of lines and adopts signals received from the line.